The skin functions as a barrier for protecting the body and inhibits the invasion of a foreign substance in organism. Specifically, the stratum corneum of the outermost layer which is directly brought in contact with the foreign substance assumes the great role as a barrier. To be sure, although the digestive tract is the same as the skin from the viewpoint of being directly brought in contact with the foreign substance, the digestive tract has no barrier of stratum corneum such as the skin, and it is rather composed of the nutritional absorptive cell having function positively taking in nutritional components from the foreign substance, namely foods. Both the skin and the digestive tract differ from each other greatly in this point.
On the other hand, the skin also has a function (a function of insensible perspiration) of discharging, and can be considered not as a simple protective membrane but an organ having adjusting function of permeating substances.
By the way, intramuscular injection, oral administration and administration from the colon by a suppository have been known as the administration procedure of a drug to organism. A percutaneous absorption method of administering from the skin has been proposed focusing attention on the above-mentioned function of the skin. According to the percutaneous absorption method, the administration is nearly indolent, the control of medication is easy, adverse reaction is hardly generated, and it is expected that the QOL (Quality Of Life) of a subject is also remarkably improved because of the convenience of administration mode. Further, isosorbide dinitrate, estradiol, tulobuterol, nicotine, clonidine, scopolamine, fentanyl, lidocaine and the like in addition to nitroglycerine have been developed as percutaneously absorptive type pharmaceuticals.
The above-mentioned percutaneously absorptive type pharmaceuticals has advanced the study of percutaneous absorption of a drug and it has been clear that there were many drugs which could not be percutaneously absorbed by any method in accordance with the proceeding.
Then, a method of instantaneously perforating extremely fine pores in the skin and introducing a drug utilizing the procedure of Electroporation which is used for introducing gene in cells; a method of Iontophoresis which introduces into the skin an ionized drug using the technique of electrophoresis; or an administration method combining these have been devised as the percutaneous absorption method next generation instead of the procedure of simply diffusing and absorbing a drug from the stratum corneum into the skin in conventional methods.
As a means for perforating fine pores in the skin in like manner as the Electroporation, a MicroPatch method of bringing a pad with numerous tiny needles in contact with the skin and injecting a drug from the stinging needles site has been proposed.
To illustrate the MicroPatch method more specifically, a pad for endermism used in the MicroPatch method is equipped with a plural number of solid-core thick and short needles (made of silicon, a metal, or a plastic) with acicular pyramids of 10 to 50 μm and a reservoir for drug solution. When it is used, the above-mentioned needles sting the skin, gaps are broadened by vibrating the contact plane of the needles with the skin by a vibrator (100 MHz to 2000 MHz), and the drug solution from the above-mentioned reservoir is designed to be invaded into the skin from the extremely fine pore spots of the skin (for example, refer to U.S. Pat. No. 6,183,434).
As the drug administered by the MicroPatch method, insulin, morphine, α-interferon, parthyroid hormone, erythropoietin and the like are developed (Altea Therapeutics Inc., Atlanta, USA), insulin and the like have been already under the first phase of clinical test and studies for practical application are proceeding.
As the administration method, a non needle injection method which is in contrast to the above-mentioned method is also proposed. Concretely, a method of administering subcutaneously under pressuring an injection solution, or a method of using gas with high pressure by which the powder of a drug is subcutaneously beaten in under high pressure, or the like are proposed. Practically, a portion of them is already commercialized.
Although these administration methods have both merits and demerits, the MicroPatch method is a superior method from the viewpoints that it does not require specific devices and any one can easily use it.